Check for nullptr before calling OnProofVerifyDetailsAvailable

I missed this in cl/306737289.

gfe-relnote: add nullptr check to QUIC's TlsClientHandshaker, protected by ietf quic draft version flags.
PiperOrigin-RevId: 306909773
Change-Id: Id489263eb379a5c79e421ac8df4515155b60df52

quic/core/tls_client_handshaker.cc

diff --git a/quic/core/tls_client_handshaker.cc b/quic/core/tls_client_handshaker.cc
index 9e9e214..4b6c2af 100644
--- a/quic/core/tls_client_handshaker.cc
+++ b/quic/core/tls_client_handshaker.cc
@@ -35,8 +35,10 @@
   parent_->verify_result_ = ok ? ssl_verify_ok : ssl_verify_invalid;
   parent_->state_ = STATE_HANDSHAKE_RUNNING;
   parent_->proof_verify_callback_ = nullptr;
-  parent_->proof_handler_->OnProofVerifyDetailsAvailable(
-      *parent_->verify_details_);
+  if (parent_->verify_details_) {
+    parent_->proof_handler_->OnProofVerifyDetailsAvailable(
+        *parent_->verify_details_);
+  }
   parent_->AdvanceHandshake();
 }